How to install OpenVPN server on Raspberry Pi

1. Download Raspberry Pi Imager.
2. Install Raspberry Pi OS Lite on 16GB microSD card.
   Remember to click the Setting icon to enter your Wifi SSID and password as well as enabling SSH and default username:pi and password: raspberry 
3. Insert the microSD card onto the Raspberry Pi and power it up via USB power supply.
4. Get the IP address of the Raspberry Pi (via HDMI or wifi-router web-console) then configure port forwarding on the router WAN interface for TCP 443 to the IP address of the Raspberry Pi.
5. Download PuTTY and run it to connect to Raspbian via SSH (Host Name: IP Address of the Raspberry Pi then click Open and if prompted by Putty alert, click Yes) then login as:pi and password:raspberry.
6. To change default password and timezone use the following command: sudo raspi-config
7. Run curl -L https://install.pivpn.io | bash and follow the wizard to install OpenVPN server. When prompted:
   Protocol, select TCP 443 then Ok
   Custom search domain, select No
   DNS provider, select Google then Ok
   Public IP if you have static public IP address or enter your public DNS entry then Ok
   OpenVPN 2.4 can use Elliptic Curves... select Yes
   Select 256 then Ok
   Unattended Upgrades select Yes
   Reboot, select Yes
8. Use PuTTY to reconnect to Raspbian via SSH and use the following command to create an ovpn file for a client (i.e. Android mobile, iPad, etc.):
   pivpn -a
9. On Windows command prompt, run pscp pi@x.x.x.x:/home/pi/ovpns/TheClientNameEnteredOnPreviousStep.ovpn c:\temp  to copy the TheClientNameEnteredOnPreviousStep.ovpn file to local drive.
10. Transfer the TheClientNameEnteredOnPreviousStep.ovpn file via email or Google Drive to your phone or tablet and import it using OpenVPN client app.

Note:

I have tested the speed on UDP and it performed better than TCP, however in this example we use TCP 443 (similar port as https traffic) to increase success rate of connection when there's a firewall that blocks other ports (i.e. office firewall or when travelling to certain countries).

If you need to run another instance of OpenVPN server on another port (i.e. UDP 53) on the same Raspberry Pi, copy /etc/openvpn/server.conf  to the same folder (i.e. /etc/openvpn/server_udp53.conf) then edit the dev and proto and port and server subnet and save the file.
Also edit /etc/iptables/rules.v4 file to allow the new instance of OpenVPN server subnet to access the internet.

Remember to reboot Raspberry Pi afterwards using the the following command

sudo shutdown -r now